1. Field of the Invention
The present invention relates to a process for the preparation of pyrrolaldehydes by Vilsmeier synthesis and purification by high-vacuum distillation. Pyrrolaldehydes are valuable pharmacologically active compounds and are described, for example, in EP-A 287 988 and corresponding U.S. Pat. No. 5,043,348. A Vilsmeier synthesis for the preparation thereof is also already specified there.
2. Discussion of Prior Art
The synthesis of relatively large amounts of substances by the Vilsmeier process poses problems with regard to industrial practicability. Moreover, on an industrial scale, the yields are relatively low and the purities of the products do not correspond to the requirements which must be made of a pharmacological active compound. Since, in the reaction itself, large amounts of halogen-containing solvents must be used, there are, in addition, considerable problems in keeping the waste water and exhaust air clean, and also with regard to the general safety at work. The purification of the products by repeated recrystallisation is associated with considerable losses of yield and the use of relatively large amounts of solvent.